1. Field of the Invention
The present invention relates to a catalyst for use in the polymerization of alpha-olefins such as ethylene to obtain a wide molecular weight distribution in the resultant polymers and to the process of polymerizing alpha-olefins using the catalyst.
2. Related Art
It is well known that the alpha-olefins such as ethylene can be polymerized by contacting them under polymerization conditions with "Ziegler" type catalyst, i.e., a catalyst obtained by activating a Group 4b or 5b transition metal-containing catalyst component with a cocatalyst, e.g., a titanium compound activated with an organometallic compound. It is also well known that this type of catalyst produces polymer with a narrow molecular weight distribution. For many applications it is necessary and desirable to improve the toughness of the processed polymer. One method of improving toughness properties is by preparing polymers of high molecular weight (low melt index). However, at the higher molecular weights there is a corresponding and detrimental decrease in the rheological properties of the polymer. This is compensated for in practice by broadening the molecular weight distribution, which is typically obtained by using a chromium based catalyst system in place of the Ziegler type catalyst. However, in commercial operations where both types of polymer, i.e., broad and narrow molecular weight distribution, are produced the presence of both Ti based Ziegler type catalysts and chromium based catalysts are required, and operational problems may occur since each catalyst is a poison to the other. Hence it is highly desirable that "Ziegler" compatible catalysts suitable for producing broad molecular weight polymer be developed.
According to the present invention it has been found that titanium based catalysts containing hafnium in addition to titanium and prepared by a particular procedure for use as a Ziegler type catalyst component, provide a broader molecular weight distribution for polymer produced therewith and are fully compatible with other Ziegler type catalysts.
U.S. Pat. No. 4,325,835 to Hartshorn et al. discloses a component for an olefin polymerization catalyst which is the product of treating a particular support material with (a) an organomagnesium compound, (b) an aluminum compound e.g. ethyl aluminum dichloride, (c) an organometallic compound of general formula R.sub.m MX.sub.p wherein M is a metal of Groups IA, IIA, IIB, IIIB, VA or VIA of a Periodic Table, R is a hydrocarbyl or substituted hydrocarbyl group, X is a singly charged anionic ligand or a monodentate neutral ligand, m is an integer up to the highest valency of the metal M and p is O or an integer up to 2 less than the valency of the metal M, e.g., zirconium tetrabenzyl, and (d) at least one transition metal compound of Groups IVA, VA or VIA, e.g., titanium tetrachloride.
U.S. Pat. No. 4,385,161 to Caunt et al. discloses a catalyst component for polymerization of olefin monomers which is a transition metal composition which is obtained by reacting together an inert particulate material, an organic magnesium compound, a halogen-containing compound such as carbon tetrachloride, silicon tetrachloride or boron trichloride and a specified transition metal compound such as VOCl.sub.3, bis(n-butoxy) titanium dichloride or zirconium tetrabenzyl.
U.S. Pat. No. 4,396,533 to Johnstone discloses a supported Ziegler catalyst prepared by the following steps:
(A) heating together at a temperature in the range of 250.degree. to 1100.degree. C. a refractory oxide support material, such as silica, and one or more halogen-free metal derivatives which are hydrides and/or organic derivatives of the metal, such as titanium tetraisopropylate;
(B) reacting the product from (A) with one or more organometallic compounds having a general formula MR.sup.1.sub.a Q.sub.b-a wherein M is a metal atom, R.sup.1 is a hydrocarbon group, Q is a halogen or an oxyhydrocarbyl group, b is a valency of M and a is an integer from 1 to b and wherein the metal atom M is aluminum, boron, lithium, zinc or magnesium such as triethyl aluminum; and
(C) impregnating the solid product from step (B) with one or more halogen-containing transition metal compounds wherein the metal or metals comprise titanium and/or vanadium and/or zirconium such as a mixture of vanadium oxytrichloride and titanium tetrachloride.
U.S. Pat. No. 4,397,762 to Johnstone discloses a supported Ziegler catalyst prepared by the following steps, carried out under anhydrous conditions:
(A) reacting a hydroxyl groups-containing support material comprising magnesium silicate or silica and magnesia with one or more organometallic compounds having the general formula MR.sup.1.sub.a Q.sub.b-a wherein M is a metal which is aluminum, boron, lithium, zinc or magnesium, R.sup.1 is a hydrocarbyl group, Q is a halogen or an oxyhydrocarbyl group, b is the valency of M and a is an integer from 1 to b;
(B) removing unreacted organometallic compound if any, from the produced solid material; and
(C) impregnating the solid material obtained from step B with one or more halogen-containing transition metal compounds wherein the transition metal(s) comprise titanium, vanadium or zirconium.
European Patent Application No. 058,549 filed by Asaki Kasei Kogyo Kabushiki Karisha discloses a catalyst for polymerizing olefins comprising a solid catalyst component A a.hd an organometallic component B in which the component A is obtained by reacting an (1) organometallic compound and (2) a transition metal compound or mixture of compounds of Ti, V, Ti and V, and Ti and Zr in the presence of a (3) solid reaction product of (a) an organomagnesium compound and (b) a halide of B, Si, Ge, Sn, P, Sb, Bi, Zn or hydrogen chloride and optionally (c) a solid inorganic oxide.
Other references, e.g., U.S. Pat. Nos. 4,310,648, 4,356,111 and 4,399,053 issued to Shipley, et al. and U.S. Pat. No. 4,396,747 to Welch et al. disclose unsupported organomagnesium/Zn/Ti olefin polymerization catalyst, unlike the present composition which requires the use of a support material from a Group 2a, 3a, 4a, or 4b metal oxide such as, for example, dehydrated silica.
In a related, commonly assigned application of the same inventor, Ser. No. 626,172, filed June 29, 1984 it was disclosed that a titanium based catalyst containing zirconium in addition to titanium and supported on dehydrated silica, produces alpha-olefin polymers having broadened molecular weight distribution in Ziegler polymerizations. However, it has been found that as the amount of zirconium is increased, in order to obtain beneficially broader molecular weight distribution in the polymer, the hydrogen response of the catalyst is so substantially reduced, such that melt index (MI) of less than 0.003 is obtained.
It is an advantage of the present invention that a rather specific catalyst has been discovered, which demonstrates good hydrogen response, for the production of broad molecular weight (MWD&gt;10) linear polymers of alpha-olefins, in particular high density polyethylene and linear low density polyethylene. In particular, the present catalyst differs, among other reasons, from that of U.S. Pat. No. 4,325,835 in that that patent requires at least one aluminum compound; from U.S. Pat. No. 4,385,166, U.S. Pat. No. 4,396,533 and U.S. Pat. No. 4,397,762, in that a very wide and diverse variety of support materials and reactive components are taught by the references, which in the specifics produce substantially different catalysts from the present invention; from EPA No. 0058549 in that the reference discloses that a solid organomagnesium compound is present prior to the introduction of transition metal compound, the use of only zirconium and vanadium as second transition metal components, and the use of an organometallic compound (1) of lithium, magnesium, aluminum or zinc as an additional catalyst component, whereas in the present catalyst preparation a hafnium compound reacts directly with the organomagnesium compound and the organometallic compound is not used.
The unsupported catalyst are less active than the present catalyst and polymers produced with such catalyst may require deashing of the polymer to remove excess metals or operation at very high ethylene partial pressure.